This invention relates to means for increasing the transmission capacity of communications systems, particularly multiple user optical fiber systems, which entails pulse frequency division multiplexing. By utilizing the pulse modulation, i.e., the pulse repetition rate, of transmitting light sources to identify the individual signals of myriad simultaneous users, the effective throughput of local communications systems may be increased to hundreds of gigabits per second (Gb/s).
Single-mode optical fibers widely used in telecommunication networks have extensive bandwidths ranging into hundreds of GHz-krn. Despite this potential, the highest optical fiber transmission bit rates presently utilized are limited by current optoelectronic conversion technology to a range of only several Gb/s. In order to achieve maximum data capacity, or throughput, in these networks, the most practical expedient is some system of multiplexing the individual transmissions of multiple customers or users of the networks.
A number of such multiplexing systems have been implemented, notable among which is high density wavelength division multiplexing (WDM) incorporating tunable wavelength selection to distinguish individual customer signals. Although offering some promise, this approach, as well as coherent communications techniques using local optical oscillation, have exhibited significant practical and economical difficulties. Likewise, code division multiple access (CDMA) and its variants using spread-spectrum communications have attracted some attention, but in addition to being relatively slow, suffer from dispersion and other non-linear effects in optical fibers which in turn rely upon dispersion compensation techniques not yet readily available.
The pulse frequency division multiplexing (PFDM) system of the present invention, on the other hand, provides numerous advantages over those earlier proposed techniques. Pulse frequency, for example, is more simply controllable electrically than optical frequency of semiconductor laser signal sources. Further, PFDM is asynchronous in implementation and thus provides a distinct advantage over time division multiplexing systems. The invention additionally incorporates a system of low power, high speed optical gate elements by which individually identified user signal throughput may be maintained at multi-Gb/s rates within readily controllable bit-error ratios (BER).